Quantum lattice fluctuations in the one-dimensional molecular-crystal model with Coulomb repulsion.

Abstract

The effect of quantum lattice fluctuations on the ground state of a half-filled-band one-dimensional molecular-crystal model with on-site (U) and nearest-neighbor (V) Coulomb repulsion is investigated. The nonadiabatic effects due to finite phonon frequency \ensuremath{\omega} are treated through a variational polaron wave function. The electronic correlations are decoupled by a Hartree-Fock approximation. Our variational approach gives a rather good description of the continuous variation of the dimerization, which is in good agreement with that of the Monte Carlo simulations, as functions of U, V, and \ensuremath{\omega}. However, the Hartree-Fock theory predicts a fictitious long-range spin-density-wave order and a discontinuous transition between it and the charge-density-wave (CDW) state. These are somewhat different from the results of numerical simulations. Furthermore, we propose a valence-bonding (VB) state for describing the short-range antiferromagnetic correlation for intermediate values of U and V and determine the transition point between the VB state and the CDW state, which is in good agreement with that of the numerical simulations. The effect of quantum lattice fluctuations on the transition point is also discussed.